A retro-reflector can be used to reflect an incident signal back to the source. A problem with such a retro-reflector is that it has a certain reflectivity that is hard to suppress when it is not being used. This is even harder to do when being used.
Modulators incorporated into retro-reflector systems usually consist of complicated waveguide or RF components built into antenna arrays. Such arrays have been used in the past to give wide area coverage. Units built in an array form have a cost and complexity directly proportional to the array size.